What the numbers mean, and what really matters

From the Crutchfield Archives

If you're shopping for an HDTV, you've probably seen terms like "720p" and "1080p", or "1366 x 768 pixels" used to describe a television's resolution. But what exactly do those numbers mean, and what do they say about a TV's performance? In this article, we'll walk you through the basics of resolution, and give you some practical tips to help you decide how high a resolution you need for your new HDTV.

What is resolution?

The main reason high-definition TV pictures look so much sharper and clearer than regular TV is HDTV's higher resolution. In today's world of digital TVs, resolution is measured in pixels, with more pixels providing higher resolution. Old-fashioned TVs had the equivalent of around 300,000 pixels, while today's HDTVs offer one to two million — up to six times more. All those additional pixels mean a huge jump in picture quality.

The image on the left simulates the picture resolution of an old-fashioned TV, while the image on the right simulates high-definition TV. Notice the soft edges and jagged lines in the non-HD image.

When we talk about picture resolution, we're actually talking about two things: the resolution of your TV's screen and the resolution of the video source (your DVD player, cable box, etc.). Both are important, and each can affect the other in determining the quality of the picture you see. Let's take a closer look at each so you know how they relate, and how to get a good high-resolution picture.

TV screen resolution

Nearly all of today's HDTVs are "fixed-pixel displays," meaning their screens use a fixed number of pixels to produce a picture. That includes flat-panel LCD and plasma TVs, as well as front- and rear-projection types that use DLP, LCD, or LCoS technology.

All of these fixed-pixel displays have a native resolution that tells you the maximum level of image detail a TV can produce. Two of the most common resolutions are 768p and 1080p, though you may also see 720p.

You may see these same resolutions listed as "1366 x 768 pixels" or "1920 x 1080 pixels." That tells you precisely how many pixels the screen actually has: the first number is the horizontal resolution and the second number is the vertical resolution. Multiplying these two numbers gives you a screen's total pixel count. As an example, 1920 x 1080 = 2,073,600 pixels, which is usually simplified to "2 million." By comparison, 1366 x 768 = 1,049,088 pixels — slightly over one million.

These grids simulate the different-sized pixels of common TV screen resolutions, from 480i (the resolution of old-fashioned TVs) to high-definition 720p and 1080p. As resolution increases, the pixels get smaller, allowing much finer picture detail to be accurately displayed.

Now, let's move on to video source resolution. Then, we'll explain what "i" and "p" mean when you see one of those letters next to a resolution number.

Video source resolution

The two most common high-def video source resolutions are 720p and 1080i. All HDTV broadcasts, including local over-the-air broadcasts, satellite and cable signals, use one of these formats. 1080i is the most common resolution, but both formats have their benefits and limitations:

1080i has more lines and pixels to show more detail, so it's great for slow-moving programs with lots of close-ups — think Law and Order or nature documentaries on The Discovery Channel. But the "i" tells you that it's an interlaced format, which means fewer video frames per second, so it doesn't handle fast-moving video as well as 720p.

The "p" in 720p tells you it's a progressive-scan format, which means it presents fast-moving action much more cleanly. It's ideal for things like sports and action-packed video games.

What about 1080p?

These days, the most talked-about HD format is 1080p, which combines the superior resolution of 1080i with the progressive-scan smoothness of 720p. True 1080p content is still scarce, however. It's mainly available from Blu-ray players and video game consoles such as the Xbox 360™ and PS3, though you can also enjoy some 1080p content via select satellite TV programming (call your provider for details). Still, when you hear 1080p mentioned, it's generally referring to a TV's screen resolution rather than a source.

One more thing

Another key thing to understand about video source resolution is that it can also limit how good your HDTV's picture looks. If you give your TV a lower-resolution source, like a fuzzy analog cable channel, that's what you'll see — a high-def TV can't transform a poor picture into a great-looking picture. If you want to see true high-definition images on your HDTV, you'll need to feed it a high-def source — 720p, 1080i, or (in a few cases) 1080p.

What "i" and "p" mean, and how they can affect the level of picture detail

As we mentioned before, "i" stands for interlaced-scan and "p" stands for progressive-scan. These terms originated when all TVs used picture tubes, and images were "scanned" — painted across the screen line by line. Interlaced-scan images required two passes to create a complete video frame, while progressive-scan displayed the entire frame with just one pass (see illustration below). The frame rate for interlaced video is 30 frames per second while progressive-scan video is 60 frames per second.

Interlaced scan splits each video frame into two "fields," displaying all the even horizontal scan lines (2,4,6...) in 1/60th of a second, followed by the odd scan lines (1,3,5...) during the next 1/60th of a second. That means you'll see a complete video frame every 1/30th of a second.

Progressive scan, on the other hand, displays all the lines in a single sweep (1,2,3,4...). You'll see a complete frame every 1/60th of a second.

The bottom line

Today's digital TV displays are nearly all effectively progressive-scan, so interlaced and progressive are mostly relevant when describing video source signals sent to the TV. The main thing to remember is that a progressive signal has twice as much picture information as an equivalent interlaced signal, and generally looks a little more solid and stable, with on-screen motion that's more fluid.

This graph shows the total amount of picture information displayed at each resolution, per second. 1080p's combination of high screen resolution and progressive-scan frame rate allow it to deliver twice as much picture information as the other options — which means a clearer, smoother picture. Hopefully, we'll see more 1080p content soon.

What happens if your TV and video source have different resolutions?

This scenario actually happens all the time, and fortunately with today's HDTVs, you don't really need to worry about it. Whether the resolution of your video source material is low (VHS), medium (DVD), or high (HDTV), a fixed-pixel TV will always automatically convert or scale the video signal to fit the screen's native resolution. Scaling lower-quality signals to fit a TV's higher-resolution screen is often called upconversion. Upconversion works great with a good source like DVD, but it can't make snowy analog antenna reception or a noisy cable picture look flawlessly crisp and clear.

Similarly, if the incoming source has more pixels than the screen's native resolution, the video signal has to be "downconverted." It's like trying to pour 10 pounds of sugar into a 5-pound bag: you have to throw away some detail to fit the image on the screen. That's one of the reasons 1080p TVs are so popular — they can display every pixel of every available high-def resolution, so they never have to throw any detail out. But if you don't get a 1080p TV, don't worry — downconverted video can still look great. The best example is 1080i HD broadcasts that are downconverted to be viewed on 768p TVs.

What's the resolution of 3D TV?

Some 3D TVs have 720p resolution, while others offer 1080p. When you watch 2D material on a 3D TV, you'll get the same high-definition picture you'd get with a non-3D model.

Most 3D TVs allow you to enjoy your TV's full resolution when watching 3D material as well. These models use active shutter glasses, which alternately block out the left or right images in sync with your TV. Only your right eye sees the right image and only your left eye sees the left image, but each eye gets a full 720p or 1080p picture.

A few 3D TVs use passive glasses. These polarized glasses divvy up the 3D picture into right-eye and left-eye views — just like the ones used in movie theaters. This means that each eye is seeing roughly half of the full picture resolution. For example, viewing a 1080p 3D picture with passive glasses gives you a 540p left-eye picture and a 540p right-eye picture. Despite the lower resolution, viewers who find the flicker effect of active shutter glasses distracting often find passive glasses more comfortable. You can find more info about how 3D TV works in our 3D TV FAQ.

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Crutchfield Writer Biography

Steve Kindig

Steve Kindig has been an electronics enthusiast for over 30 years. He has written extensively about home and car A/V gear for Crutchfield since 1985. Steve is also a volunteer announcer in the folk department of community radio station WTJU, where he is one of the regular hosts of "Atlantic Weekly."